[Refactor] Use ScopedTestPublicSessionLoginState in code

chromeos/login/scoped_test_public_session_login_state.h

Index: chromeos/login/scoped_test_public_session_login_state.h
diff --git a/chromeos/login/scoped_test_public_session_login_state.h b/chromeos/login/scoped_test_public_session_login_state.h
index 02b0c33ca32b92ffce0f24e81d68a383b77d1c8c..d055f6766001021cc82cd2ffaccddb0ecd39bfe9 100644
--- a/chromeos/login/scoped_test_public_session_login_state.h
+++ b/chromeos/login/scoped_test_public_session_login_state.h
@@ -9,13 +9,17 @@
 
 namespace chromeos {
 
-// A class to start and shutdown public session state for a test.
+// A class to start and shutdown public session state for a test. Only one
+// instance is allowed to exist at a given time. To be instantiated on the stack
+// (so it nicely cleans up after going out of scope).
 class ScopedTestPublicSessionLoginState {
  public:
   ScopedTestPublicSessionLoginState();
   ~ScopedTestPublicSessionLoginState();
 
  private:
+  bool needs_shutdown_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(ScopedTestPublicSessionLoginState);
 };